By the use of density gradient sedimentation of disrupted cells in the zonal ultracentrifuge followed by iso pycnic separation with a ficol barrier it is possible to obtain a relatively purified membrane preparation in two simple steps. The purified membrane will be attacked with proteases or lipid solvents under controlled conditions to yield soluble products which will be tested for a variety of biological activities. Rat, mouse, and human livers as well as stable tissue culture cell lines can all be studied in this manner. Through the use of neuraminidase a number of new antigenic specificities can be shown on the lymphocyte membrane. Antibodies to these ligands are present in all allo-immune sera tested to date. These antibodies are confusing attempts to specifically label HLA antigens with ferritin. We propose to study the localization of ferritin labelled antibody, purified by absorption with enzyme treated cells and eluted from platelets, on plain and neuraminidase treated lymphocytes.